Honestly, when it comes to the shows represented by fighters that aren't the main stars, I think each of them still make sense:
1. Nigel and Oblina are both major characters of their respective shows (and Nigel in particular is arguably one of the more memorable parts of the show, even if only in a memetic sense).
2. Not only is Helga a major character (and more known for fighting compared to Arnold, who was only really a fighter in one episode), but she arguably becomes just as much a main character as Arnold himself.
3. With Reptar, while he isn't a main character (or even a proper character in-universe), he's still a pretty iconic part of the show, so I'm fine with him representing Rugrats (plus, as others have mentioned, it's possible someone in the dev team has an issue with beating up a literal baby, so they went with the Godzilla expy instead).
Heck, another show I'd like to see represented is ChalkZone, and I honestly wouldn't mind seeing Snap as a playable fighter.